(a) Field of the Invention
The present invention relates to a signal transmitting apparatus. More particularly, the present invention relates to a bias modulation apparatus and an apparatus and method for transmitting a wideband mobile communication signal using the same.
(b) Description of the Related Art
At present, mobile communication systems have been developed from an existing code division multiple access (CDMA) system to a system having a multi-carrier modulation method such as orthogonal frequency division multiplexing (OFDM) for a high data transmission speed. For example, WIMAX, WIBro, and 3G long term evolution (LTE) systems select an OFDM modulation method. The OFDM system has a drawback that a peak to average power ratio (PAPR) of a transmission signal is high by the summation of carriers.
The most important characteristics in a transmitting apparatus of a mobile communication system are linearity and efficiency, but linearity and efficiency are in a trade-off relationship and thus it is difficult to satisfy both.
Currently, a method that has been discussed in order to increase efficiency while satisfying both characteristics may include an envelope elimination and restoration method (EER) of inputting a phase signal to a switching power amplifier using a polar coordinate and applying envelope curve information to a bias modulation unit of a power amplifier, and an envelope tracking method (ET) of modulating bias of a power amplifier while applying a general complex in-phase/quadrature (I/Q) signal, which is not a phase signal, as an input signal of the power amplifier.
In the EER method, because a signal in which an envelope curve is removed becomes the input of a power amplifier, amplification efficiency is maximized, but there is a drawback that the power amplifier should be matched to a wideband due to a band extension problem of the input phase signal. Further, because the EER method is very sensitive to a time error between an envelope curve information signal and a phase output signal, it is very difficult to form hardware.
In order to compensate for this, amplification efficiency is theoretically reduced a little but an up-conversion signal of a complex I/Q signal is used as an input signal, and a structure using an ET method that modulates bias of a power amplifier according to a size of an input signal has been in the spotlight.
Because both the EER method and the ET method should modulate bias to correspond to an envelope curve signal, overall transmission efficiency is represented by multiplication of efficiency of a bias modulation unit and overall efficiency of a power amplifier.η=ηbias modulation unit×ηpower amplifier  [Equation1]
Here, η represents transmission efficiency of a system, ηbias modulation unit represents efficiency of the bias modulation unit, and ηpower amplifier represents efficiency of a power amplifier.
Therefore, in a power amplifier of a structure using an EER or ET method, efficiency of the bias modulation unit becomes an important parameter that determines efficiency of an entire system, and a method of maximizing the efficiency is necessary.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.